High bypass turbofan engines are widely used for high performance aircraft that operate at subsonic speeds. These engines have a large fan placed at the front of the engine to produce greater thrust and reduce specific fuel consumption. The fan serves to compress incoming air, a portion of which is then delivered to the combustion chamber, with a larger portion being bypassed to the rear of the engine to generate additional engine thrust. The fan is circumscribed by a fan casing that must be capable of containing and minimizing damage to the engine from the remote event of a fan blade that is released from its hub during engine operation. For this reason, fan casings are equipped with specialized blade containment structures that serve to minimize structural damage to the engine as well as the aircraft to which the engine is mounted.
Various materials and configurations for blade containment structures have been proposed. Steel is well suited for blade containment on the basis of its mechanical properties, and particularly its toughness (strain to failure). However, a significant drawback to the use of steel in aerospace applications is its density. Consequently, thin steel containment structures coupled with a wrap formed of KEVLAR.RTM. or another fiber-reinforced polymer material have been developed. While reducing weight, these containment structures are characterized by significantly higher manufacturing costs. Containment structures formed of relatively lightweight metals such as aluminum alloys have also been used, though they do not provide the level of high toughness and other desirable mechanical properties possible with steel.
An additional consideration for blade containment structures is the natural frequency of the casing and the avoidance of blade/case interaction. The frequency of steel containment structures has typically been increased above blade/case interaction frequencies by the inclusion of rings that are integral with or bolted to the structure. Frequency-altering measures such as integral or bolt-on rings have also been required with aluminum containment structures. For steel containment structures coupled with fiber-reinforced polymer wraps, a honeycomb structure between the steel component and the wrap has been used to increase the natural frequency of the casing assembly. In addition to additional material, and manufacturing and assembly costs, each of the above modifications for addressing blade/case interaction incurs the penalty of unwanted weight, space and cost.
From the above, it can be seen that improvements in blade containment through material selection based on mechanical properties and the structural requirements for avoiding blade/case interaction have combined to increase the weight and cost of manufacturing high bypass turbofan engines. It would be desirable if a blade containment structure were available that exhibited the blade containment capabilities of steel casings but without the weight penalty associated with steel, while also maintaining fan casing natural frequencies at acceptable margins to avoid blade/case interactions.